I watched the video. Hadn't before my last comment. It sure looks like some type of surging problem to me. Now my guess is that it's not a belt problem. It's a cyclical supercharger resistance problem is my bet.

Do you have the ability take the belt off yourself? If you do - take it off and rotate the supercharger input shaft by hand. Feel whether it has a spot of high resistance as it goes around or whether it spins with roughly the same resistance throughout a few rotations of the pulley.

The next place you should consider looking, and I'd bet this is where you find a problem, is for a leak in the vacuum bypass or a problem at the vacuum bypass valve (like a sticky butterfly valve - which is my armchair quarterback diagnosis #1). If you have a vacuum leak or a problem with the diaphragm in the vacuum bypass solenoid mechanism it could cause something like what you're experiencing if there's condition where the vacuum causes it to close the bypass ->engine surges from extra air from the supercharger -> ECU closes throttle body slightly more to stabilize rpm -> manifold vacuum increases -> supercharger bypass opens -> rpms drop -> ECU opens throttle body -> manifold vacuum drops -> bypass closes some-> and the cyclical balancing keeps going round and round. Just spitballing here on the cycle that would happen - but suffice it to say that a vacuum leak somewhere could very well cause a problem and this may be a symptom of that. I'd place my bet on it being a vacuum leak somewhere.

Tell Magnusson they can send me a free one for diagnosing the problem from a 10 second video.

Good luck!